1. Field of Invention
The present invention relates to a high frequency power amplifier that is used in mobile communication devices.
2. Description of Related Art
High frequency power amplifiers that are used in cell phones, for example, have an amplification device such as a transistor, an input matching circuit for efficiently inputting a high frequency signal to the amplification device, and an output matching circuit for efficiently outputting the high frequency signal from the amplification device. The matching circuits have a capacitance and a high frequency matching device such as an inductor or microstrip line, and are typically rendered as a packaged chip mounted on a circuit board. A semiconductor chip containing the high frequency amplification device is also mounted on the circuit board, and combined these components render a high frequency power amplifier module.
Cell phones have come to offer a wide range of functions including multiband signal transmission capabilities and multimode functions for handling different modulation signals. Battery size has also decreased as cell phones have become smaller, thus requiring higher efficiency high frequency power amplifiers in order to assure sufficient communication time, and there is a need to emphasize efficiency during low power operation instead of only near the maximum output level. As known from the literature, optimizing the efficiency of the high frequency power amplifier requires matching the input and output impedance under various frequency, output, and other conditions, and plural individual high frequency power amplifiers having matching circuits with optimized impedance are required in order to provide multiband and multimode compatibility.
U.S. Pat. No. 6,281,748 (corresponding to Japanese Laid-open Patent Publication No. 2001-251202) teaches an arrangement having a control device affording such compatibility.
FIG. 19 shows an example of the prior art. The matching circuit on the output side of the last amplification device includes a variable capacitance 300 and capacitance 301, a switching diode 302 connected in series to the capacitance, and an arrangement including an inductor 303, a bypass capacitor 304, and a resistance 305 on the path connected to the control circuit, and the impedance of the output load circuit 306 can be varied by controlling these devices. If the devices 300 to 305 are rendered as chips mounted on a circuit board, the area of the power amplification module including the matching circuit is increased as shown in FIG. 20. The area is further increased and the module becomes even more complicated if sophisticated control techniques for multiband switching and output power switching are also incorporated.
The present invention provides a high frequency power amplifier that enables easily adjusting and switching the impedance while reducing the area of the power amplification module and assuring high performance and low cost.